The present invention relates generally to gamma titanium aluminide (TiAl) alloys having improved castability in the sense of improved grain structure. More particularly, it relates to castings of niobium doped TiAl which achieves fine grain microstructure and a set of improved properties with the aid of combined niobium and boron additives.
In forming a casting, it is generally desirable to have highly fluid properties in the molten metal to be cast. Such fluidity permits the molten metal to flow more freely in a mold and to occupy portions of the mold which have thin dimensions and also to enter into intricate portions of the mold without premature freezing. In this regard, it is generally desirable that the liquid metal have a low viscosity so that it can enter portions of the mold having sharp corners and so that the cast product will match very closely the shape of the mold in which it was cast.
Another desirable feature of cast structures is that they have a fine microstructure, that is a fine grain size, so that the segregation of different ingredients of an alloy is minimized. This is important in avoiding metal shrinking in a mold in a manner which results in hot tearing. The occurrence of some shrinkage in a casting as the cast metal solidifies and cools is quite common and quite normal. However, where significant segregation of alloy components occurs, there is a danger that tears will appear in portions of the cast article which are weakened because of such segregation and which are subjected to strain as a result of the solidification and cooling of the metal and of the shrinkage which accompanies such cooling. In other words, it is desirable to have the liquid metal sufficiently fluid so that it completely fills the mold and enters all of the fine cavities within the mold, but it is also desirable that the metal once solidified be sound and not be characterized by weak portions developed because of excessive segregation or internal hot tearing.
With regard to the titanium aluminide itself, it is known that as aluminum is added to titanium metal in greater and greater proportions, the crystal form of the resultant titanium aluminum composition changes. Small percentages of aluminum go into solid solution in titanium and the crystal form remains that of alpha titanium. At higher concentrations of aluminum (including about 25 to 30 atomic percent) and intermetallic compound Ti.sub.3 Al forms and it has an ordered hexagonal crystal form called alpha-2. At still higher concentrations of aluminum (including the range of 50 to 60 atomic percent aluminum) another intermetallic compound, TiAl, is formed having an ordered tetragonal crystal form called gamma. The gamma titanium aluminides are of primary interest in the subject application.
The alloy of titanium and aluminum having a gamma crystal form and a stoichiometric ratio of approximately 1, is an intermetallic compound having a high modulus, low density, a high thermal conductivity, a favorable oxidation resistance, and good creep resistance. The relationship between the modulus and temperature for TiAl compounds to other alloys of titanium and in relation to nickle base superalloys is shown in FIG. 1. As is evident from the Figure, the gamma TiAl has the best modulus of any of the titanium alloys. Not only is the gamma TiAl modulus higher at higher temperature, but the rate of decrease of the modulus with temperature increase is lower for gamma TiAl than for the other titanium alloys. Moreover, the gamma TiAl retains a useful modulus at temperatures above those at which the other titanium alloys become useless. Alloys which are based on the TiAl intermetallic compound are attractive, light-weight materials for use where high modulus is required at high temperatures and where good environmental protection is also required.
One of the characteristics of gamma TiAl which limits its actual application to such uses is a brittleness which is found to occur at room temperature. Another of the characteristics of gamma TiAl which limits its actual application is a relatively low fluidity of the molten composition. This low fluidity limits the castability of the alloy particularly where the casting involves thin wall sections and intricate structure having sharp angles and corners. Improvements of the gamma TiAl intermetallic compound to enhance fluidity of the melt as well as the attainment of fine microstructure in a cast product are very highly desirable in order to permit more extensive use of the cast compositions at the higher temperatures for which they are suitable. When reference is made herein to a fine microstructure in a cast TiAl product, the reference is to the microstructure of the product in the as-cast condition.
It is recognized that if the product is forged or otherwise mechanically worked following the casting, the microstructure can be altered and may be improved. However, for applications in which a cast product is useful, the microstructure must be attained in the product as cast and not through the application of supplemental mechanical working steps.
What is also sought and what is highly desirable in a cast product is a minimum ductility of more than 0.5%. Such a ductility is needed in order for the product to display an adequate integrity. A minimum room temperature strength for a composition to be generally useful is about 50 ksi or about 350 MPa. However, materials having this level of strength are of marginal utility and higher strengths are often preferred for many applications. The stoichiometric ratio of gamma TiAl compounds can vary over a range without altering the crystal structure. The aluminum content can vary from about 50 to about 60 atom percent. However, the properties of gamma TiAl compositions are subject to very significant changes as a result of relatively small changes of 1% or more in the stoichiometric ratio of the titanium and aluminum ingredients. Also, the properties are similarly affected by the addition of relatively small amounts of ternary and quaternary elements as additives or as doping agents.